Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

Achieving a selective 2 e(-) or 4 e(-) oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso-phenyls each bearing a bulky ortho-amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers alpha beta alpha beta and alpha alpha alpha alpha catalyze ORR with n=2.10 and 3.75 (n is the electron number transferred per O-2), respectively, but alpha alpha beta beta and alpha alpha alpha beta show poor selectivity with n=2.89-3.10. Isomer alpha beta alpha beta catalyzes 2 e(-) ORR by preventing a bimolecular O-2 activation path, while alpha alpha alpha alpha improves 4 e(-) ORR selectivity by improving O-2 binding at its pocket, a feature confirmed by spectroscopy methods, including O K-edge near-edge X-ray absorption fine structure. This work represents an unparalleled example to improve 2 e(-) and 4 e(-) ORR by tuning only steric effects without changing molecular and electronic structures.

Automated summary

This study demonstrates the control of ORR selectivity of Co porphyrins by tuning steric effects, with αβαβ isomer catalyzing 2 e(-) ORR and αααα isomer improving 4 e(-) ORR selectivity. This finding expands the pathway for modulating molecular selectivity in catalytic performance.